In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, introduction of dual connectivity has been discussed (see Non Patent Document 1).
In the dual connectivity, a user terminal simultaneously establishes a connection with a plurality of cells. In this case, each cell is managed by different base stations respectively.
In the dual connectivity, of the plurality of base stations configured to establish a connection with the user terminal, only one base station (hereinafter, called “master base station”) establishes an RRC connection with the user terminal. On the other hand, of the plurality of base stations, another base station (hereinafter, “secondary base station”) provides an additional radio resource to the user terminal without establishing the RRC connection with the user terminal.